1. Field of the Invention
This invention relates to an outboard motor and more particularly to an improved exhaust arrangement for outboard motors having four-cycle internal combustion engines.
2. Description of Related Art
An outboard motor, because of its compact nature, generally employs an exhaust system that includes an expansion chamber in the drive shaft housing and exhausts through a submerged discharge port. The expansion chamber performs the function of assisting and silencing the exhaust gases, as does the underwater exhaust gas discharge.
Outboard motors often also include an above water, idle exhaust gas discharge through which exhaust gases are expelled under idle and low speed running conditions. The idle exhaust gas discharge reduces back pressure in the exhaust system when the engine is running at low speeds and the associated watercraft is not planing.
A renewed interest in employing four-cycle engines as the power plants in outboard motors has recently arisen in order to address environmental concerns. A four-cycle engine, unlike a two-cycle engine though, requires a source of lubricant to hold the lubricant that has recirculated through the engine for lubricating purposes. In order to provide adequate storage capacity and to maintain a low profile for the outboard motor, the lubricant tank is generally positioned in the drive shaft housing below the engine. This positioning for the lubricant tank exposes it to the exhaust gases and thus presents a problem of heating the lubricant in the lubricant tank.
Outboard motors also conventionally employ an open-loop cooling system that draws cooling water from the body of water in which the outboard motor is operated (e.g., a lake or an ocean). The cooling water often is introduced into the exhaust system in order to cool the exhaust gases and the exhaust system, and to silence exhaust gases. The exhaust system thence returns the cooling water to the body of water.
If the engine temporarily becomes disabled, the pressure within the engine cylinder(s) can become less than the pressure within the exhaust system, and the resulting negative pressure differential may be sufficient to draw the introduced cooling water into the engine cylinder(s). Water invasion inside the engine can cause corrosion and damage the intake and exhaust valves of the four-cycle engine. In addition, because the supply of cooling water may temporarily stop, the temperature of the exhaust system can increase and consequently damage the exhaust system and surrounding components.
An aspect of the present invention involves an improved exhaust and cooling system arrangement for an outboard motor that adequately cools the exhaust system proximate to the lubricant tank during normal running conditions (both at high and low speeds), as well as continues to cool a portion of the exhaust system in proximity to the lubricant tank if the engine becomes disabled. The exhaust and cooling systems also are arranged to inhibit an influx of coolant into the engine through the exhaust system if the engine becomes temporarily disabled.
The outboard motor comprises a power head and a drive shaft housing depending from the power head. The power head contains a four-cycle, internal combustion engine that includes at least one coolant passage. A cooling system runs coolant through the coolant passage. An expansion chamber is located in the drive shaft housing, and an exhaust pipe depends below the engine and communicates with the expansion chamber to deliver exhaust gases to the expansion chamber. A lubricant tank for the engine is disposed in the drive shaft housing above the expansion chamber and in proximity to the exhaust pipe. A collection space is formed between at least a portion of the exhaust pipe and a portion of the lubricant tank. The collection space communicates with the cooling system such that coolant fills at least a portion of the collection space.
Further aspects, features, and advantages of the present invention will become apparent from the detailed description of the preferred embodiment which follow.